librados: add a way to get a count of omap vals in an iterator
We currently have functions like rados_read_op_omap_get_vals2 that hand back an iterator to a userland caller. There is no way to get a count of the objects inside the iterator however. Add a rados_omap_iter_size function that will tell you the number of omap elements in the returned iterator structure.
This is needed for ganesha so that it can preallocate structures with an appropriate size to hold all of the elements returned in the iterator.